cache_flat_mutation_reader: Use rows insertion sugar
cache_flat_mutation_reader: Use rows insertion sugar

When inserting a rows_entry via unique_ptr the ptr inquestion
can be pushed as is, the intrusive btree code releases the
pointer (to be exception safe) itself. This makes the code
a bit shorter and simpler.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla